Where low voltage subsets are normally used it is known to provide a special purpose dialler chip having a mode output signal to indicate to its associated transmission circuit the mode of operation in order to bring the transmission circuit into a tone dialling mode in which the voltage across the subsets line terminals during dialling is increased such that sufficient power is available to the dialler chip to generate dial signals.
Where high voltage subsets are normally used, the voltage across the line terminals provides sufficient power for the dialler chip to generate dial signals, and therefore a mode output signal is not required.